Bisphenols in the molten condition, at elevated temperatures in the range of above about 100.degree. to about 350.degree. C. or higher, i.e. 400.degree. C., are known to decompose to form, in general, simpler phenolic substances i.e. monohydroxy phenols, such as phenol together with other decomposition products such as monohydroxy phenols containing an ethylenically unsaturated aliphatic side chain, i.e. alkenyl phenols.
For example, bisphenol A, i.e. 2,2-(4,4'dihydroxy diphenyl) propane, is known to decompose on heating to form equimolar proportions of phenol and p-isopropylidene phenol.
On further heating, the aforementioned thermal decomposition products of bisphenols, particularly the aforementioned phenol product containing an aliphatic unsaturated substituent can, form complex non-volatile compounds.
The above-described thermal decomposition products of the bisphenols, even in small concentrations, have an adverse effect on compositions, particularly polymers, e.g. linear aromatic polyesters, prepared from the bisphenols. In polyesters, for example, the adverse effect of the aforementioned decomposition product impurities include lowering of the polyester molecular weight.
Since the exact mechanism of thermal decomposition of bisphenol in a non-reactive environment for the bisphenol is complex and is not known with certainty, the selection of suitable bisphenol thermal stabilizer agents has been particularly empirical. For example, British Pat. No. 890,432, to Farbenfabriken Bayer, discloses secondary and tertiary alkaline earth phosphates, stannous oxalate, tin powder, mixtures of tin dioxide and tin powder, terephthalic acid, isophthalic acid, oxalic acid, adipic acid, sebacic acid, boron trioxide, antimony trioxide or mixtures thereof as stabilizer additives for molten bisphenols.
Also British Pat. No. 1,022,583, to Monsanto Corporation, discloses the aforementioned oxalic acid as stabilizer and additionally discloses citric acid, tartaric acid or alkali metal or ammonium salts of the foregoing acids as bisphenol stabilizer additives.
British Pat. No. 924,607, of B. E. Jennings, issued Apr. 24, 1963, assigned to Imperial Chemicals Industries, Ltd. teaches addition of an aliphatic titanate ester such as tetra n-butyl titanate, to mixtures of bisphenol and a dicarboxylic acid diaryl ester to catalyze transesterification polymerization of the mixtures to linear aromatic polyester of the bisphenol and dicarboxylic acid. This patent does not teach addition of the titanate ester to the bisphenol as stabilizer or teach bisphenol-titanate ester mixtures in a non-reactive environment for the bisphenol.